Operational amplifiers (“opamps”) are basic circuits utilized in a wide range of electronic circuits. In addition to amplification and buffering, opamps are typically utilized to implement operations such as summing, integration, multiplication, and others. Typical opamp applications include comparators, oscillators, filters, sample and hold circuits, and instrumentation amplifiers.
Opamps are often subject to an inherent input-referred offset voltage. Generally, when the voltages at the differential inputs of the opamp are equal, the output voltage should theoretically be at the mid-supply voltage. In actual applications, a slight offset in the output voltage from the mid-supply voltage occurs when the input voltages are equal. This voltage offset is commonly called the input-referred offset voltage. Additionally, opamp transistors typically generate flicker noise during switching.
In many more sensitive applications, reducing input-referred offset and flicker noise are important design considerations. Hence, one particular technique that has been utilized for addressing the problems of input-referred offset and flicker noise is chopper stabilization. In chopper stabilization, the signal of interest at the input of one or more stages of an opamp is modulated or “chopped” at a high frequency. Typically, the chopping frequency is selected to be at least twice the frequency of the band of the signal of interest to avoid aliasing. At the output of the chopper-stabilized stage, the signal of interest is demodulated back into the original signal band by a second chopping operation. This second chopping also modulates any inherent offset and/or flicker (1/f) noise out of the frequency band of the signal of interest.
While chopper stabilization advantageously minimizes the effects of input-referred offset and flicker noise, chopper stabilization can also reduce amplifier gain and/or produce chopping artifacts in the opamp output. In some applications, such as audio amplification, neither a significant reduction in gain nor the introduction of chopping artifacts in the opamp output signal is normally acceptable. For such noise and gain sensitive applications, new chopper stabilization techniques are required.